Fluoropolymers, i.e. polymers having a fluorinated backbone, have been long known and have been used in a variety of applications because of several desirable properties such as heat resistance, chemical resistance, weatherability, UV-stability etc . . . . The various fluoropolymers are for example described in “Modern Fluoropolymers”, edited by John Scheirs, Wiley Science 1997. The fluoropolymers may have a partially fluorinated backbone, generally at least 40% by weight fluorinated, or a fully fluorinated backbone. Particular examples of fluoropolymers include polytetrafluoroethylene (PTFE), copolymers of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) (FEP polymers), perfluoroalkoxy copolymers (PFA), ethylenetetrafluoroethylene (ETFE) copolymers, terpolymers of tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride (THV) and polyvinylidene fluoride polymers (PVDF).
The fluoropolyrners may be used to coat substrates to provide desirable properties thereto such as for example chemical resistance, weatherability, water- and oil repellency etc . . . . For example aqueous dispersions of fluoropolymer may be used to coat kitchen ware, to impregnate fabric or textile e.g. glass fabric, to coat paper or polymeric substrates. For sake of economy and convenience, the fluoropolymer dispersions will typically have between 35% by weight and 70% by weight of fluoropolymer solids.
A frequently used method for producing aqueous dispersions of fluoropolymers involves aqueous emulsion polymerization of one or more fluorinated monomers usually followed by an upconcentration step to increase the solids content of the raw dispersion obtained after the emulsion polymerization. The aqueous emulsion polymerization of fluorinated monomers generally involves the use of a fluorinated surfactant. Frequently used fluorinated surfactants include perfluorooctanoic acids and salts thereof, in particular ammonium perfluoiooctanoic acid. Further fluorinated surfactants used include perfluoropolyether surfactants such as disclosed in EP 1059342, EP 712882, EP 752432, EP 816397, U.S. Pat. Nos. 6,025,307, 6,103,843 6,126,849. Still further surfactants that have been used are disclosed in U.S. Pat. Nos. 5,229,480, 5,763,552, 5,688,884, 5,700,859, 5,804,650, 5,895,799, WO 00/22002 and WO 00/71590.
Most of these fluorinated surfactants have a low molecular weight, i.e. a molecular weight of less than 1000 g/mol. Recently, such low molecular weight fluorinated compounds have raised environmental concerns. Accordingly, measures have been taken to either completely eliminate the fluorinated low molecular weight surfactants from aqueous dispersion or at least to minimize the amount thereof in an aqueous dispersion. For example, WO 96/24622 and WO 97/17381 disclose an aqueous emulsion polymerization to produce fluoropolymers whereby the polymerization is carried out without the addition of fluorinated surfactant. U.S. Pat. No. 4,369,266 on the other hand discloses a method whereby part of fluorinated surfactant is removed through ultrafiltration. In the latter case, the amount of fluoropolymer solids in the dispersion is increased as well, i.e. the dispersion is upconcentrated while removing fluorinated surfactant. WO 00/35971 further discloses a method in which the amount of fluorinated surfactant is reduced by contacting the fluoropolymer dispersion with an anion exchanger.
Since the solids content of the raw dispersions immediately after emulsion polymerization is usually in the range of up to 30% by weight, the raw dispersions are subjected to an upconcentration process so as to increase the solids content thereof. In order to preserve the stability of the dispersion, the upconcentration typically is carried out in the presence of a stabilizer, in particular a non-ionic surfactant that acts as a stabilizer.
However, when fluoropolymer dispersions that contain no or only a small amount of fluorinated low molecular weight surfactant are upconcentrated, it was found that a viscosity increase results which may be unacceptable. Moreover, the stability of the upconcentrated dispersions may under certain conditions be inferior to dispersions in which the amount of low molecular weight fluorinated surfactant is higher.
In applicant's WO patent application no. US02/25114 filed Aug. 6, 2002, the aforementioned increase in viscosity was reduced by adding a non-fluorinated anionic surfactant to the dispersion. As a result, the viscosity of the dispersion could be adjusted to a level that is comparable to that of a dispersion containing large amounts of the fluorinated surfactant. Although the application discloses several possible non-ionic surfactants, all of the examples were carried out with TRITON™ X-100 (an ethoxylated alkylphenol surfactant), which is a commonly used non-ionic surfactant for fluoropolymer dispersions. This surfactant has the disadvantage of being objected to from an environmental point of view. Further, this surfactant may cause undesirable discoloring in coatings produced from dispersions containing this surfactant.
In U.S. Pat. No. 6,153,688 dispersions of polytetrafluoroethylene (PTFE) have been disclosed that contain between 35 and 65% by weight of PTFE and between 2 and 10% by weight relative to PTFE of a polyoxyethylene alkyl ether non-ionic surfactant. The latter is selected such that the cloud point thereof is more than 45° C. and the amount of ethylene oxide is between 65 and 70% by weight. It is taught that such dispersions offer the advantage of being free of aromatic group containing surfactants while still having good coating properties as the viscosity at room temperature is low. But, the dispersions are apparently prepared in the known way with the use of fluorinated surfactant and therefore are believed to have a high amount of fluorinated surfactant.
It would now be desirable to find further aqueous dispersions of fluoropolymer, in particular PTFE, that are free of or substantially free of low molecular weight fluorinated surfactant and that are free of non-ionic surfactant having an aromatic group. Also, it is desirable that such dispersions have good coating properties in particular for impregnation of fabrics. Desirably, the good coating properties are obtained over a commercially feasible range of operating conditions, in particular with respect to ambient conditions. Further, desirably, the dispersion has good film forming properties comparable or better as dispersions that contain the low molecular weight fluorinated surfactant in large amounts.